The present invention is related to the field of printers and copiers and more particularly to printers or copiers that utilize fusers, intermediate transfer members and/or elements that function as both fusers and intermediate transfer members.
Printers and copiers are well known. Modern copiers that utilize powder or liquid toners comprising toner particles to form visible images generally form a latent electrostatic image on an image forming surface (such as a photoreceptor), develop the image utilizing a toner (such as the aforementioned powder or liquid toners) to form a developed image and transfer the developed image to a final substrate. The transfer may be direct, i.e., the image is transferred directly to the final substrate from the image forming surface, or indirect, i.e., the image is transferred to the final substrate via one or more intermediate transfer members.
In general, the image on the final substrate must be fused and fixed to the substrate. This step is achieved in most copiers and printers by heating the toner image on the substrate. In some copiers and printers the fusing and fixing of the image is performed simultaneously with the transfer of the image to the substrate. This is achieved by utilizing a heated intermediate transfer member to perform the transfer and by pressing the intermediate transfer member against the final substrate. This combination of heat and pressure softens the toner particles and fixes them to the substrate.
These processes and fixers, intermediate transfer members other components and liquid toners suitable for carrying them out and printers utilizing these structures and processes are described in detail in U.S. Pat. Nos. 4,945,387; 5,047,808; 5,028,964; 5,089,856; 5,157,238; 5,286,948; 5,335,054; 5,497,222; 5,554,476; and 5,636,349; and PCT patent publications WO 96/17277, WO 97/07433 and PCT patent applications PCT/IL98/00235 and PCT/IL98/00553, the disclosures of all of which are incorporated herein by reference.
Particular reference is made to U.S. Pat. Nos. 5,047,808; 5,554,476 and 5,636,349 which describe a number of attributes of preferred intermediate transfer members suitable for liquid toner imaging.
U.S. Pat. No. 5,047,808 describes an intermediate transfer member comprised of a rigid core and an overlying intermediate transfer blanket. As described in the patent, a preferred intermediate transfer member provides a first transfer of images from an image bearing surface to the intermediate transfer member and a second transfer of the images from the intermediate transfer member to the final substrate. While both first and second transfers are performed under pressure, second transfer (which includes fixing and fusing of the image to the substrate) is performed under much higher pressure than first transfer. The patent teaches that the deformation per unit pressure during first transfer should be much lower than during second transfer. In other words, the intermediate transfer member should be xe2x80x9charderxe2x80x9d for second transfer.
U.S. Pat. No. 5,335,054 provides a particularly advantageous method of achieving this desired characteristic of the intermediate transfer member. This patent describes an intermediate transfer member having two types of layers which contribute to this effect. In particular, the preferred intermediate transfer member as described in this patent has a soft, thin conforming layer, preferably formed of a soft polymer, and a sponge layer underlying the soft conforming layer. These layers provide conformance of the intermediate transfer member with the surface of the image bearing surface at low pressure and relatively low deformation and the desired stiffness of the intermediate transfer member under higher pressure conditions. Advantageously, a plurality of sponge and/or conforming layers are used to provide greater control over the compressibility profile of the member at first and second transfer.
U.S. Pat. No. 5,636,349 describes another desirable characteristic of intermediate transfer members. As described in this patent, the intermediate transfer member should be heated to a temperature at which the image on it adheres to the substrate. While the member is still pressing against the substrate the member is cooled sufficiently such that the cohesion of the image increases to such an extent that the image cohesion forces are greater than those causing adhesion to the member. When these conditions are met, the image is transferred in its entirety from the intermediate transfer member to the final substrate without leaving any appreciable toner residue on the intermediate transfer member.
It can be appreciated that this combination of requirements (and other requirements which have not been mentioned above) places very tight limitations on intermediate transfer members. While intermediate transfer members as described in the prior art can meet these requirements, the transfer parameters must be tightly controlled and the operating window available for these processes is limited. In state of the art systems the required transfer temperatures are provided by heating the drum on which the blanket is mounted, such that the image transfer surface is heated to a required temperature of 90 to 110 degrees Celsius. Higher or lower temperatures are also useful, depending on the polymers used in the toner particles, the carrier liquid used and the speed of the printing process. Since the blanket needs a sponge layer to provide some of the compressibility requirements of the member, and since sponges generally have high thermal impedance, the back of the blanket is much hotter than its transfer surface, often as much as 60-70 degrees hotter.
This results in severe requirements on the materials used in the blanket, which must not only meet the stringent operating requirements mentioned above, but must also do so under high temperature, often much higher than the temperatures required for the actual transfer process. Furthermore, it has been found that the sponge layer is susceptible to damage from paper misfeeds or jams. When a number of sheets are fed together or jams occur, the sponge is sometimes compressed past its recovery point.
Furthermore, it has been found that intermediate transfer members exhibit short term memory effects under certain conditions. These effects manifest themselves in slightly different transfer characteristics for areas which carried an image on a previous transfer from areas which did not (background areas). It is believed that the memory effect is caused by variations in surface temperature on the transfer surface and/or by uneven absorption of carrier liquid from the liquid toner by a surface transfer layer of the transfer member. PCT patent publication WO/96/13760 and U.S. Pat. No. 5,592,269 provide at least partial solutions to these problems, at the cost of some additional system and/or toner complexity.
Reference is also made to U.S. Pat. No. 5,286,948, which describes a fusing apparatus and method utilizing a thin membrane as a flusing element. The membrane is mounted on two end elements to form a cylindrical drum of which the membrane forms the cylindrical surface. This element, which is generally too thin to support itself, especially during transfer, is supported by gas pressure within the drum and/or by mechanically applied pressure on the end elements to tension the membrane. It should be noted that the gas pressure itself also provides pressure on the end elements to tension the membrane.
It should be understood that while the above background art inventions have been discussed with respect to liquid toner electrophoretic imaging machines, many of the principles described and some of the structure is applicable to powder toner machines and to offset printers utilizing non-electrified inks.
It is an object of some preferred embodiments of the invention to provide an intermediate transfer member or fuser of improved design and performance.
Preferably, the intermediate transfer member or fuser comprises a thin membrane, as an image transfer and/or flusing element, that is mounted on two end elements to form a cylindrical drum, of which the membrane forms the cylindrical surface. The membrane, which may be too thin to support itself, especially during transfer, is supported by gas pressure within the drum and optionally by mechanically applied pressure on the end elements to tension the membrane. A gas pressure of about two to three atmospheres has been found to be suitable for supporting the membrane. Preferably, a relatively simple intermediate transfer blanket is mounted on the outside of the cylindrical surface.
As indicated in the above background of the invention, the prior art method of providing the compression characteristics is to include in the blanket at least one sponge layer, one conforming layer, one conducting layer and means for electrically connecting to the conductive layer, all of which make the blanket relatively complicated to manufacture and relatively expensive. Such a blanket is expensive to manufacture, has a low heat conductivity and is susceptible to damage from paper missfeeds. It has been found that, fortuitously, when a pressure supported membrane is used as a support for the blanket it is not necessary to provide a sponge layer beneath the conforming layer to achieve the required compression characteristic. It has been found that the deformation of the membrane under external pressure has characteristics sufficiently similar to that of the prior art sponge layer that, with optional changes in the conforming layer, no (or at most a very thin) sponge layer is required.
Under these conditions, with the sponge layer removed, a thinner, much less expensive blanket may be used. This blanket has a much lower thermal resistance and thus, the drum itself need not be heated to as high a temperature as required in the prior art. In particular, it has been found that a temperature differential as low as 20 or 30 degrees Celsius may be sufficient. This lower temperature requirement allows for use of lower temperature materials for adhesives and other components of the blanket and for higher reliability of the blanket as a whole.
In one aspect of some preferred embodiments of the invention, an image transfer member is provided whose temperature is stabilized. In a preferred embodiment of the invention the image transfer member comprises a drum and the temperature is stabilized by incorporating a relatively small quantity of liquid within the drum, preferably in contact with a portion of the inside surface of the cylindrical thin membrane.
The inclusion of the liquid in the drum has a number of positive effects on the operation of the intermediate transfer member. One important unexpected result is a greatly improved stability of the characteristics of the intermediate transfer member.
As indicated above, a membrane drum of the prior art could be used as an intermediate transfer member. One of the advantages of such an intermediate transfer member is its low heat capacity which allows for short warm-up time. However, when an intermediate transfer blanket is mounted on such a drum, some of the problems of such blankets are exacerbated. In particular, use of a low thermal capacity drum makes it more difficult to measure and control the temperate of the drum. In addition, the temperature varies over surface portions of the drum as a function of the angular and axial position of portion, often to an unacceptable degree.
An important limitation of such an intermediate transfer member is that it has a relatively high short term memory. It is believed that local variations in the surface temperature are naturally induced by evaporation of carrier liquid associated with the image. It is believed that areas having toner particles, and associated liquid, cool preferentially between first and second transfer due to the evaporation of carrier liquid associated with the toner particles. While the non-toner areas are also covered with a thin layer of carrier liquid, the evaporation of this thin layer does not reduce the temperature as much as does the evaporation of a greater amount of carrier liquid from the toner covered areas. While it is possible to reduce this effect by increasing the temperature of the drum, this is not an optimal solution for the problem, inter alia because too high a drum temperature may interfere with first transfer or even damage the photoreceptor.
Whatever the source of the short term memory, the inclusion of a liquid, such as water or oil, in the drum appears to sharply reduce the effect. In practice, the liquid forms a xe2x80x9cpoolxe2x80x9d at the lowest portion of the drum. When, during rotation, the drum surface passes through this pool, the temperature of the surface is xe2x80x9cresetxe2x80x9d to the temperature of the liquid. Furthermore, it is believed that the liquid forms a thin coating of liquid on the inner surface of the membrane. This coating provides a greater heat capacity to the drum, even outside the region of the pool of liquid and reduces the deleterious effect of evaporation of carrier liquid. Generally, only small amounts of liquid are required, of the order of 5% of the volume of the interior of the drum, although lesser or greater amounts may be advantageously used. This small amount of liquid does not change the warm-up time of the drum to an unacceptable degree.
An additional requirement for membrane type drums is the provision of pressure within the drum to support the drum. This was provided, in the prior art reference described above, by sealing the interior of the drum and providing an inlet valve through which the interior was pressurized by a gas. However, while such a system did operate as required, inevitable leakage required monitoring and periodic re-pressurization.
In an especially preferred embodiment of the invention, the liquid in the drum is water. Use of water as the liquid provides an automatic pressure and temperature stabilization feature to the drum. It has been found that, fortuitously, at 120-130 degrees Celsius, the vapor pressure of water is about two to three atmospheres. Thus, if the water (and thus the membrane) is heated to this temperature, a temperature which provides a suitable surface temperature for the transfer surface of the intermediate transfer blanket, the internal pressure is also in an optimum range for image transfer. It should be understood that for powder toner systems a higher temperature and pressure are required, such that use of water for the filling is believed to be suitable for powder toner systems as well. Furthermore, the temperature and pressure desired may vary depending on the speed of the printing process and the polymer and carrier liquid used in the toner.
It should also be noted that when materials are dissolved in the water, the vapor pressure is reduced. Thus, where a higher temperature is desired for a particular pressure, a suitable amount of material is added to the water to reduce the pressure. Alternatively or additionally a mixture of liquids may be used to control the viscosity of the liquid and/or the vapor pressure.
In preferred embodiment of the invention, the drum contains air at at least one atmosphere. This filling with air is desirable to avoid collapse of the drum when it is cooled. Preferably a one way valve is provided such that the pressure in the drum never falls below the outside pressure.
There is thus provided, in accordance with a preferred embodiment of the invention, intermediate transfer member apparatus for transferring visible images from a first surface to a second surface or a fuser for fusing an image on a surface, comprising:
a cylindrical member secured between two round end plates to form a cylindrical structure; and
a liquid incorporated within the cylindrical structure.
Preferably, the member includes a heater which heats the liquid. Preferably, the heater heats the liquid and the cylindrical member to a temperature between about 110 degrees Celsius and about 140 degrees Celsius, more preferably, between about 115 degrees Celsius and about 135 degrees Celsius and most preferably, between about 120 degrees Celsius and about 130 degrees Celsius.
In a preferred embodiment of the invention, the heater is a radiant heater situated in the interior of the cylindrical structure. Alternatively, the heater is a conduction heater placed in a pool of the liquid in the cylindrical structure.
In a preferred embodiment of the invention, the cylindrical member forms a seal at the end plates and wherein said cylindrical surface is supported by gas pressure internal to the cylindrical structure. Preferably, the gas pressure is equal to between about 2 and about 3 atmospheres. Preferably, the gas pressure comprises vapor pressure of the liquid.
In a preferred embodiment of the invention, the liquid comprises water.
In a preferred embodiment of the invention, the apparatus includes a one way valve which allows gas to pass from the exterior of the cylindrical structure to the interior thereof.
In a preferred embodiment of the invention, the liquid comprises an oil.
In a preferred embodiment of the invention, the liquid comprises a mixture of different liquids. Alternatively or additionally the liquid has a vapor pressure affecting material dissolved in it.
In a preferred embodiment of the invention, the apparatus includes a transfer surface on an external cylindrical surface of the cylindrical structure. Preferably, the transfer surface is comprised in a transfer blanket attached to the cylindrical member. Preferably, the transfer blanket comprises at least one solid elastomer layer. Preferably, the transfer blanket does not include any sponge material.
Preferably, the transfer blanket includes an exterior transfer surface and when the transfer surface is heated from within the cylindrical structure to a temperature of 100 degrees Celsius, the cylindrical member is at a temperature no more than 30 degrees Celsius and more preferably no more than 20 degrees Celsius higher than that of the transfer surface.
In a preferred embodiment of the invention, the cylindrical member is a membrane having a thickness of between 50 and 250 micrometers, more preferably between 100 and 200 micrometers and more preferably, 125 micrometers or greater.
Preferably, the cylindrical member is comprised of nickel.
In a preferred embodiment of the invention, the interior of the cylindrical structure is hollow and wherein the liquid fills less than the entire hollow, preferably less than half the hollow, more preferably less than 25% or 10% of the hollow. In a preferred embodiment of the invention only about 5% of the hollow is filled with the liquid.
In a preferred embodiment of the invention, the liquid contacts an interior surface of the cylindrical member. Preferably, as the member rotates, the liquid is carried along the interior surface as a film.
There is further provided, in accordance with a preferred embodiment of the invention, printing apparatus comprising:
an image forming surface on which a visible image is formed; and
an intermediate transfer member, according to the invention, which receives the image from the image forming surface and transfers it to another surface.
Preferably, the visible image is a toner image. The toner image is preferably either a liquid or powder toner image.